Optical film

ABSTRACT

Birefringent optical films have a Brewster angle (the angle at which reflectance of p-polarized light goes to zero) which is very large or is nonexistent. This allows for the construction of multilayer mirrors and polarizers whose reflectivity for p-polarized light decreases slowly with angle of incidence, are independent of angle of incidence, or increase with angle of incidence away from the normal. As a result, multilayer films having high reflectivity (for both planes of polarization for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers) over a wide bandwidth, can be achieved.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/527,452 filed Mar. 17,2000, now U.S. Pat. No. 6,296,927, which is a divisional of Ser. No.09/145,371, filed on Sep. 2, 1998 (U.S. Pat. No. 6,117,530), which is acontinuation of Ser. No. 08/402,041, filed Mar. 10, 1995 (U.S. Pat. No.5,882,774), which is a continuation-in-part of application Ser. No.08/359,436, filed Dec. 20, 1994 (abandoned), which is acontinuation-in-part of application Ser. No. 08/171,239, filed Dec. 21,1993 (abandoned).

BACKGROUND

The present invention relates to optical films useful, e.g., aspolarizers and/or mirrors.

Light-reflecting devices based upon multiple polymeric layers are known.Examples of such devices include polarizers made of alternatingpolymeric layers in which the layers have different refractive indices.

SUMMARY

The optical properties and design considerations of birefringent opticalfilms described herein allow the construction of multilayer stacks forwhich the Brewster angle (the angle at which reflectance of p-polarizedlight goes to zero) is very large or is nonexistant. This allows for theconstruction of multilayer mirrors and polarizers whose reflectivity forp-polarized light decreases slowly with angle of incidence, areindependent of angle of incidence, or increase with angle of incidenceaway from the normal. As a result, multilayer films having highreflectivity (for both s and p polarized light for any incidentdirection in the case of mirrors, and for the selected direction in thecase of polarizers) over a wide bandwidth, can be achieved.

Briefly, in one aspect the present invention provides a multilayeredpolymer film comprising layers of a crystalline or semi-crystallinenaphthalene dicarboxylic acid polyester, for example a 2,6-polyethylenenaphthalate (“PEN”) or a copolymer derived from ethylene glycol,naphthalene dicarboxylic acid and some other acids such as terephthalate(“co-PEN”), with a positive stress optical coefficient, i.e. uponstretching its index of refraction in the stretch direction increases,having an average thickness of not more than 0.5 microns; and layers ofa selected second polymer, for example a polyethylene terephthalate(“PET”) or a co-PEN, having an average thickness of not more than 0.5microns. Preferably, after stretching of the films of this invention inat least one direction, the layers of said naphthalene dicarboxylic acidpolyester have a higher index of refraction associated with at least onein-plane axis than the layers of the second polymer. The film of thisinvention can be used to prepare multilayer films having an averagereflectivity of at least 50% over at least a 100 nm wide band.

In another aspect, the present invention provides a multilayered polymerfilm comprising layers of a crystalline or semi-crystalline polyester,for example a PET, having an average thickness of not more than 0.5microns: and layers of a selected second polymer, for example apolyester or a polystyrene, having an average thickness of not more than0.5 microns: wherein said film has been stretched in at least onedirection to at least twice that direction's unstretched dimension. Thefilm of this invention can be used to prepare multilayer films having anaverage reflectivity of at least 50% over at least a 100 nm wide band.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained with reference to the drawings.

FIGS. 1a and 1 b are diagrammatical views of the polarizer of thepresent invention.

FIG. 2 is a graphical view illustrating the refractive indicescharacteristics of the PEN and coPEN layers of the present invention.

FIG. 3 is a graphical view of computer simulated data of percenttransmission of a 50-layer PEN/coPEN film stack based on the indicesshown in FIG. 2.

FIG. 4 is a graphical view of computer simulated data of percenttransmission of an equally biaxially stretched 300-layer PEN/coPETmirror.

FIG. 5 is a graphical view of percent measured transmission of a51-layer I.R. polarizer of the present invention with the first orderpeak near 1,300 nm.

FIG. 6 is a graphical view of percent measured transmission of eight51-layer polarizers of the present invention laminated together.

FIG. 7 is a graphical view of percent measured transmission of a204-layer polarizer of the present invention.

FIG. 8 is a graphical view of percent measured transmission of two204-layer polarizers of the present invention laminated together.

FIG. 9 is a schematic view of an overhead projector of the presentinvention.

FIG. 10 shows a two layer stack of films forming a single interface.

FIGS. 11 and 12 show reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.60.

FIG. 13 shows reflectivity versus angle curves for a uniaxialbirefringent system in a medium of index 1.0.

FIGS. 14, 15 and 16 show various relationships between in-plane indicesand z-index for a uniaxial birefringent system.

FIG. 17 shows off axis reflectivity versus wavelength for two differentbiaxial birefringent systems.

FIG. 18 shows the effect of introducing a y-index difference in abiaxial birefringent film with a large z-index difference.

FIG. 19 shows the effect of introducing a y-index difference in abiaxial birefringent film with a smaller z-index difference.

FIG. 20 shows a contour plot summarizing the information from FIGS. 18and 19:

FIGS. 21-26 show optical performance of multilayer mirrors given inExamples 3-6;

FIGS. 27-31 show optical performance of multilayer polarizers given inExamples 7-11:

FIG. 32 shows the optical performance of the multilayer mirror given inExample 12;

FIG. 33 shows the optical performance of the AR coated multilayerreflective polarizer of Example 13;

FIG. 34 shows the optical performance of the multilayer reflectivepolarizer of Example 14; and

FIGS. 35a-c show optical performance of multilayer polarizers given inExample 15.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention as illustrated in FIGS. 1a and 1 b includes amultilayered polymeric sheet 10 having alternating layers of acrystalline naphthalene dicarboxylic acid polyester such as 2.6polyethylene naphthalate (PEN) 12 and a selected polymer 14 useful as areflective polarizer or mirror. By stretching PEN-selected polymer overa range of uniaxial to biaxial orientation, a film is created with arange of reflectivities for differently oriented plane-polarizedincident light. If stretched biaxially, the sheet can be stretchedasymmetrically along orthogonal axes or symmetrically along orthogonalaxes to obtain desired polarizing and reflecting properties.

For the polarizer, the sheet is preferably oriented by stretching in asingle direction and the index of refraction of the PEN layer exhibits alarge difference between incident light rays with the plane ofpolarization parallel to the oriented and transverse directions. Theindex of refraction associated with an in-plane axis (an axis parallelto the surface of the film) is the effective index of refraction forplane-polarized incident light whose plane of polarization is parallelto that axis. By oriented direction is meant the direction in which thefilm is stretched. By transverse direction is meant that directionorthogonal in the plane of the film to the direction in which the filmis oriented.

PEN is a preferred material because of its high positive stress opticalcoefficient and permanent birefringence after stretching, with therefractive index for polarized incident light of 550 nm wavelengthincreasing when the plane of polarization is parallel to the stretchdirection from about 1.64 to as high as about 1.9. The differences inrefractive indices associated with different in-plane axes exhibited byPEN and a 70-naphthalate/30-terephthalate copolyester (coPEN) for a 5:1stretch ratio are illustrated in FIG. 2. In FIG. 2, the data on thelower curve represent the index of refraction of PEN in the transversedirection and the coPEN while the upper curve represents the index ofrefraction of PEN in the stretch direction. PEN exhibits a difference inrefractive index of 0.25 to 0.40 in the visible spectrum. Thebirefringence (difference in refractive index) can be increased byincreasing the molecular orientation. PEN is heat stable from about 155°C. up to about 230° C. depending upon shrinkage requirements of theapplication. Although PEN has been specifically discussed above as thepreferred polymer for the birefringent layer, polybutylene naphthalateis also a suitable material as well as other crystalline naphthalenedicarboxylic polyesters. The crystalline naphthalene dicarboxylicpolyester should exhibit a difference in refractive indices associatedwith different in-plane axes of at least 0.05 and preferably above 0.20.

Minor amounts of comonomers may be substituted into the naphthalenedicarboxylic acid polyester so long as the high refractive index in thestretch direction(s) is not substantially compromised. A drop inrefractive index (and therefore decreased reflectivity) may be counterbalanced by advantages in any of the following: adhesion to the selectedpolymer layer, lowered temperature of extrusion, better match of meltviscosities, better match of glass transition temperatures forstretching. Suitable monomers include those based on isophthalic,azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthaleneicarboxylic, 2,6-naphthalene dicarboxylic or cyclohexanedicarboxylicacids.

The PEN/selected polymer resins of the present invention preferably havesimilar melt viscosities so as to obtain uniform multilayer coextrusion.The two polymers preferably have a melt viscosity within a factor of 5at typical shear rates.

The PEN and the preferred selected polymer layers of the presentinvention exhibit good adhesion properties to each other while stillremaining as discrete layers within the multilayered sheet.

The glass transition temperatures of the polymers of the presentinvention are compatible so adverse effects such as cracking of one setof polymer layers during stretching does not occur. By compatible ismeant that the glass transition temperature of the selected polymer islower than the glass transition temperature of the PEN layer. The glasstransition temperature of the selected polymer layer temperature may beslightly higher than the glass transition temperature of the PEN layer,but by no more than 40° C.

Preferably, the layers have a ¼ wavelength thickness with different setsof layers designed to reflect different wavelength ranges. Each layerdoes not have to be exactly ¼ wavelength thick. The overridingrequirement is that the adjacent low-high index film pair have a totaloptical thickness of 0.5 wavelength. The bandwidth of a 50-layer stackof PEN/coPEN layers having the index differential indicated in FIG. 2,with layer thicknesses chosen to be a ¼ wavelength of 550 nm, is about50 nm. This 50-layer stack provides roughly a 99 percent averagereflectivity in this wavelength range with no measurable absorption. Acomputer-modeled curve showing less than 1 percent transmission (99percent reflectivity) is illustrated in FIG. 3. FIGS. 3-8 include datacharacterized as percent transmission. It should be understood thatsince there is no measurable absorbance by the film of the presentinvention that percent reflectivity is approximated by the followingrelationship:

100−(percent transmission)=(percent reflectivity).

The preferred selected polymer layer 14 remains isotropic in refractiveindex and substantially matches the refractive index of the PEN layerassociated with the transverse axis as illustrated in FIG. 1a. Lightwith its plane of polarization in this direction will be predominantlytransmitted by the polarizer while light with its plane of polarizationin the oriented direction will be reflected as illustrated in FIG. 1b.

The reflective polarizer of the present invention is useful in opticalelements such as ophthalmic lenses, mirrors and windows. The polarizeris characterized by a mirror-like look which is considered stylish insunglasses. In addition. PEN is a very good ultraviolet filter,absorbing ultraviolet efficiently up to the edge of the visiblespectrum. The reflective polarizer of the present invention would alsobe useful as a thin infrared sheet polarizer.

For the polarizer, the PEN/selected polymer layers have at least oneaxis for which the associated indices of refraction are preferablysubstantially equal. The match of refractive indices associated withthat axis, which typically is the transverse axis, results insubstantially no reflection of light in that plane of polarization. Theselected polymer layer may also exhibit a decrease in the refractiveindex associated with the stretch direction. A negative birefringence ofthe selected polymer has the advantage of increasing the differencebetween indices of refraction of adjoining layers associated with theorientation axis while the reflection of light with its plane ofpolarization parallel to the transverse direction is still negligible.Differences between the transverse-axis-associated indices of refractionof adjoining layers after stretching should be less than 0.05 andpreferably less than 0.02. Another possibility is that the selectedpolymer exhibits some positive birefringence due to stretching, but thiscan be relaxed to match the refractive index of the transverse axis ofthe PEN layers in a heat treatment. The temperature of this heattreatment should not be so high as to relax the birefringence in the PENlayers.

The preferred selected polymer for the polarizer of the presentinvention is a copolyester of the reaction product of a naphthalenedicarboxylic acid or its ester such as dimethyl naphthalate ranging from20 mole percent to 80 mole percent and isophthalic or terephthalic acidor their esters such as dimethyl terephthalate ranging from 20 molepercent to 80 mole percent reacted with ethylene glycol. Othercopolyesters within the scope of the present invention have theproperties discussed above and have a refractive index associated withthe transverse axis of approximately 1.59 to 1.69. Of course, thecopolyester must be coextrudable with PEN. Other suitable copolyestersare based on isophthalic, azelaic, adipic, sebacic, dibenzoic,terephthalic, 2.7-naphthalene dicarboxylic, 2.6-naphthalene dicarboxylicor cyclohexanedicarboxylic acids. Other suitable variations in thecopolyester include the use of ethylene glycol, propane diol, butanediol, neopentyl glycol, polyethylene glycol, tetramethylene glycol,diethylene glycol, cyclohexanedimethanol, 4-hydroxy diphenol, propanediol, bisphenol A, and 1.8-dihydroxy biphenyl, or1.3-bis(2-hydroxyethoxy)benzene as the diol reactant. A volume averageof the refractive indices of the monomers would be a good guide inpreparing useful copolyesters. In addition, copolycarbonates having aglass transition temperature compatible with the glass transitiontemperature of PEN and with a refractive index associated with thetransverse axis of approximately 1.59 to 1.69 are also useful as aselected polymer in the present invention. Formation of the copolyesteror copolycarbonate by transesterification of two or more polymers in theextrusion system is another possible route to a viable selected polymer.

To make a mirror, two uniaxially stretched polarizing sheets 10 arepositioned with their respective orientation axes rotated 90°, or thesheet 10 is biaxially stretched. In the latter case, both PEN refractiveindices in the plane of the sheet increase and the selected polymershould be chosen with as low of a refractive index as possible toreflect light of both planes of polarization. Biaxially stretching themultilayered sheet will result in differences between refractive indicesof adjoining layers for planes parallel to both axes thereby resultingin reflection of light in both planes of polarization directions.Biaxially stretching PEN will increase the refractive indices associatedwith those axes of elongation from 1.64 to only 1.75, compared to theuniaxial value of 1.9. Therefore to create a dielectric mirror with 99percent reflectivity (and thus with no noticeable iridescence) a lowrefractive index coPET is preferred as the selected polymer. Opticalmodeling indicates this is possible with an index of about 1.55. A300-layer film with a 5 percent standard deviation in layer thickness,designed to cover half of the visible spectrum with six overlappingquarterwave stacks, has the predicted performance shown in FIG. 4. Agreater degree of symmetry of stretching yields an article that exhibitsrelatively more symmetric reflective properties and relatively lesspolarizing properties.

If desired, two or more sheets of the invention may be used in acomposite to increase reflectivity, optical band width, or both. If theoptical thicknesses of pairs of layers within the sheets aresubstantially equal, the composite will reflect, at somewhat greaterefficiency, substantially the same band width and spectral range ofreflectivity (i.e., “band”) as the individual sheets. If the opticalthicknesses of pairs of layers within the sheets are not substantiallyequal, the composite will reflect across a broader band width than theindividual sheets. A composite combining mirror sheets with polarizersheets is useful for increasing total reflectance while still polarizingtransmitted light. Alternatively, a single sheet may be asymmetricallybiaxially stretched to produce a film having selective reflective andpolarizing properties.

The preferred selected polymer for use in a biaxially stretched mirrorapplication is based on terephthalic, isophthalic, sebacic, azelaic orcyclohexanedicarboxylic acid to attain the lowest possible refractiveindex while still maintaining adhesion to the PEN layers. Naphthalenedicarboxylic acid may still be employed in minor amounts to improve theadhesion to PEN. The diol component may be taken from any that have beenpreviously mentioned. Preferably the selected polymer has an index ofrefraction of less than 1.65 and more preferably an index of refractionof less than 1.55.

It is not required that the selected polymer be a copolyester orcopolycarbonate. Vinyl polymers and copolymers made from monomers suchas vinyl naphthalenes, styrenes, ethylene, maleic anhydride, acrylates,methacrylates, might be employed. Condensation polymers other thanpolyesters and polycarbonates might also be useful, examples include:polysulfones, polyamides, polyurethanes, polyamic acids, polyimides.Naphthalene groups and halogens such as chlorine, bromine and iodine areuseful in increasing the refractive index of the selected polymer to thedesired level (1.59 to 1.69) to substantially match the refractive indexof PEN associated with the transverse direction for a polarizer.Acrylate groups and fluorine are particularly useful in decreasingrefractive index for use in a mirror.

FIG. 9 illustrates the use of the present invention as a hot mirror inan overhead projector 30. The projector 30 is a transmissive-typeprojector, and has many features of a conventional overhead projector,including a base 32 and a projection head 34. The projection head 34 isattached to the base 32 by an arm (not shown), which may be raised orlowered thereby moving the head 34 toward or away from the base 32, byconventional adjustment means. The base 32 includes a light source 36, apower supply (not shown) for the light source 36, and appropriateoptical components such as a mirror 38 for directing the light toward aprojection stage area 40. The stage area 40 in a conventional overheadprojector includes a transparent sheet such as glass typically having atleast one Fresnel lens integrally formed therein for focusing lighttoward the head 34. If a transparency having a visual image is placed onthe stage 40, the image is collected And projected such as to a nearbyprojection screen or surface by conventional optics such as a mirror 42and lens 44 located within the head 34.

A mirror 46 of the present invention is advantageously used in theoverhead projector 30 to reflect the heat-producing infrared energy fromthe light source 36 while transmitting visible light. When used toreflect infrared energy, the mirror 46 is used as a hot mirror. This isespecially important for incandescent light sources where about 85percent of the emitted energy is in the infrared wavelength. Theinfrared energy, if uncontrolled, can cause excessive heating of densetransparencies or LCD projection panels that are placed on theprojection stage 40. When used as a hot mirror, the mirror 46 isnormally positioned between the light source 36 and the projection stage40. The mirror 46 can be a separate element or the mirror can be appliedto an optical component as a coating in the light path between the lightsource and the projection stage.

Alternatively, the mirror 46 can be used in the overhead projector 30 asa cold mirror, that is a mirror that reflects visible light, whiletransmitting infrared energy. The mirror of the present invention mayalso be positioned as a folding mirror (not shown) between the lightsource 36 and the projection stage 40. Reflectance of a multilayer coldmirror can easily approach 95 percent for visible light. The mirror ofthe present invention can be applied as a cold mirror coating to aspherical concave reflector such as reflector 38 that is placed behindthe light source 36 to collect and redirect visible light emitted fromthe light source while transmitting infrared energy.

Orientation of the extruded film was done by stretching individualsheets of the material in heated air. For economical production,stretching may be accomplished on a continuous basis in a standardlength orienter, tenter oven, or both. Economies of scale and linespeeds of standard polymer film production may be achieved therebyachieving manufacturing costs that are substantially lower than costsassociated with commercially available absorptive polarizers.

Lamination of two or more sheets together is advantageous, to improvereflectivity or to broaden the bandwidth, or to form a mirror from twopolarizers. Amorphous copolyesters are useful as laminating materials,with VITEL Brand 3000 and 3300 from the Goodyear Tire and Rubber Co. ofAkron, Ohio, noted as materials that have been tried. The choice oflaminating material is broad, with adhesion to the sheets 10, opticalclarity and exclusion of air being the primary guiding principles.

It may be desirable to add to one or more of the layers, one or moreinorganic or organic adjuvants such as an antioxidant, extrusion aid,heat stabilizer, ultraviolet ray absorber, nucleator, surface projectionforming agent, and the like in normal quantities so long as the additiondoes not substantially interfere with the performance of the presentinvention.

Optical Behavior and Design Considerations of Multilayer Stacks

The optical behavior of a multilayer stack 10 such as that shown abovein FIGS. 1a and 1 b will now be described in more general terms.

The optical properties and design considerations of multilayer stacksdescribed below allow the construction of multilayer stacks for whichthe Brewster angle (the angle at which reflectance goes to zero) is verylarge or is nonexistent. This allows for the construction of multilayermirrors and polarizers whose reflectivity for p polarized light decreaseslowly with angle of incidence, are independent of angle of incidence,or increase with angle of incidence away from the normal. As a result,multilayer stacks having high reflectivity for both s and p polarizedlight over a wide bandwidth, and over a wide range of angles can beachieved.

The average transmission at normal incidence for a multilayer stack,(for light polarized in the plane of the extinction axis in the case ofpolarizers, or for both polarizations in the case of mirrors), isdesirably less than 50% (reflectivity of 0.5) over the intendedbandwidth. (It shall be understood that for the purposes of the presentapplication, all transmission or reflection values given include frontand back surface reflections). Other multilayer stacks exhibit loweraverage transmission and/or a larger intended bandwidth, and/or over alarger range of angles from the normal. If the intended bandwidth is tobe centered around one color only, such as red, green or blue, each ofwhich has an effective bandwidth of about 100 nm each, a multilayerstack with an average transmission of less than 50% is desirable. Amultilayer stack having an average transmission of less than 10% over abandwidth of 100 nm is also preferred. Other exemplary preferredmultilayer stacks have an average transmission of less than 30% over abandwidth of 200 nm. Yet another preferred multilayer stack exhibits anaverage transmission of less than 10% over the bandwidth of the visiblespectrum (400-700 nm). Most preferred is a multilayer stack thatexhibits an average transmission of less than 10% over a bandwidth of380 to 740 nm. The extended bandwidth is useful even in visible lightapplications in order to accommodate spectral shifts with angle, andvariations in the multilayer stack and overall film caliper.

The multilayer stack 10 can include tens, hundreds or thousands oflayers, and each layer can be made from any of a number of differentmaterials. The characteristics which determine the choice of materialsfor a particular stack depend upon the desired optical performance ofthe stack.

The stack can contain as many materials as there are layers in thestack. For ease of manufacture, preferred optical thin film stackscontain only a few different materials. For purposes of illustration,the present discussion will describe multilayer stacks including twomaterials.

The boundaries between the materials, or chemically identical materialswith different physical properties, can be abrupt or gradual. Except forsome simple cases with analytical solutions, analysis of the latter typeof stratified media with continuously varying index is usually treatedas a much larger number of thinner uniform layers having abruptboundaries but with only a small change in properties between adjacentlayers.

Several parameters may affect the maximum reflectivity achievable in anymultilayer stack. These include basic stack design, optical absorption,layer thickness control and the relationship between indices ofrefraction of the layers in the stack. For high reflectivity and/orsharp bandedges, the basic stack design should incorporate opticalinterference effects using standard thin film optics design. Thistypically involves using optically thin layers, meaning layers having anoptical thickness in the range of 0.1 to 1.0 times the wavelength ofinterest. The basic building blocks for high reflectivity multilayerfilms are low/high index pairs of film layers, wherein each low/highindex pair of layers has a combined optical thickness of ½ the centerwavelength of the band it is designed to reflect. Stacks of such filmsare commonly referred to as quarterwave stacks.

To minimize optical absorption, the preferred multilayer stack ensuresthat wavelengths that would be most strongly absorbed by the stack arethe first wavelengths reflected by the stack. For most clear opticalmaterials, including most polymers, absorption increases toward the blueend of the visible spectrum. Thus, it is preferred to tune themultilayer stack such that the “blue” layers are on the incident side ofthe multilayer stack.

A multilayer construction of alternative low and high index thick films,often referred to as a “pile of plates”, has no tuned wavelengths norbandwidth constraints, and no wavelength is selectively reflected at anyparticular layer in the stack. With such a construction, the bluereflectivity suffers due to higher penetration into the stack, resultingin higher absorption than for the preferred quarterwave stack design.Arbitrarily increasing the number of layers in a “pile of plates” willnot always give high reflectivity, even with zero absorption. Also,arbitrarily increasing the number of layers in any stack may not givethe desired reflectivity, due to the increased absorption which wouldoccur.

The relationships between the indices of refraction in each film layerto each other and to those of the other layers in the film stackdetermine the reflectance behavior of the multilayer stack at any angleof incidence, from any azimuthal direction. Assuming that all layers ofthe same material have the same indices, then a single interface of atwo component quarterwave stack can be analyzed to understand thebehavior of the entire stack as a function of angle.

For simplicity of discussion, therefore, the optical behavior of asingle interface will be described. It shall be understood, however,that an actual multilayer stack according to the principles describedherein could be made of tens, hundreds or thousands of layers. Todescribe the optical behavior of a single interface, such as the oneshown in FIG. 10, the reflectivity as a function of angle of incidencefor s and p polarized light for a plane of incidence including thez-axis and one in-plane optic axis will be plotted.

FIG. 10 shows two material film layers forming a single interface, withboth immersed in an isotropic medium of index no. For simplicity ofillustration, the present discussion will be directed toward anorthogonal multilayer birefringent system with the optical axes of thetwo materials aligned, and with one optic axis (z) perpendicular to thefilm plane, and the other optic axes along the x and y axis. It shall beunderstood, however, that the optic axes need not be orthogonal, andthat nonorthogonal systems are well within the spirit and scope of thepresent invention. It shall be further understood that the optic axesalso need not be aligned with the film axes to fall within the intendedscope of the present invention.

The reflectivity of a dielectric interface varies as a function of angleof incidence, and for isotropic materials, is different for p and spolarized light. The reflectivity minimum for p polarized light is dueto the so called Brewster effect, and the angle at which the reflectancegoes to zero is referred to as Brewster's angle.

The reflectance behavior of any film stack, at any angle of incidence,is determined by the dielectric tensors of all films involved. A generaltheoretical treatment of this topic is given in the text by R. M. A.Azzam and N. M. Bashara. “Ellipsometry and Polarized Light”, publishedby North-Holland, 1987.

The reflectivity for a single interface of a system is calculated bysquaring the absolute value of the reflection coefficients for p and spolarized light, given by equations 1 and 2, respectively. Equations 1and 2 are valid for uniaxial orthogonal systems, with the axes of thetwo components aligned. $\begin{matrix}{r_{pp} = \frac{{{n2z}*{n2o}\sqrt{\left( {{n1z2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}} - {{n1z}*{n1o}\sqrt{\left( {{n2z2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}}}{{{n2z}*{n2o}\sqrt{\left( {{n1z2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}} + {{n1z}*{n1o}\sqrt{\left( {{n2z2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}}}} & \left. 1 \right) \\{r_{ss} = \frac{\sqrt{\left( {{n1o2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)} - \sqrt{\left( {{n2o2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}}{\sqrt{\left( {{n1o2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)} + \sqrt{\left( {{n2o2} - {{no2}\quad \sin \quad 2\quad \theta}} \right)}}} & \left. 2 \right)\end{matrix}$

where θ is measured in the isotropic medium.

In a uniaxial birefringent system, n1x=n1y=n1o, and n2x=n2y=n2o.

For a biaxial birefringent system, equations 1 and 2 are valid only forlight with its plane of polarization parallel to the x-z or y-z planes,as defined in FIG. 10. So, for a biaxial system, for light incident inthe x-z plane, n1o=n1x and n2o=n2x in equation 1 (for p-polarizedlight), and n1o=n1y and n2o=n2y in equation 2 (for s-polarized light).For light incident in the y-z plane, n1o=n1y and n2o=n2y in equation 1(for p-polarized light), and n1o=n1x and n2o=n2x in equation 2 (fors-polarized light).

Equations 1 and 2 show that reflectivity depends upon the indices ofrefraction in the x, y (in-plane) and z directions of each material inthe stack. In an isotropic material, all three indices are equal, thusnx=ny=nz. The relationship between nx, ny and nz determine the opticalcharacteristics of the material. Different relationships between thethree indices lead to three general categories of materials: isotropic,uniaxially birefringent, and biaxially birefringent. Equations 1 and 2describe biaxially birefringent cases only along the x or y axis, andthen only if considered separately for the x and y directions.

A uniaxially birefringent material is defined as one in which the indexof refraction in one direction is different from the indices in theother two directions. For purposes of the present discussion, theconvention for describing uniaxially birefringent systems is for thecondition nx=ny≠nz. The x and y axes are defined as the in-plane axesand the respective indices, nx and ny, will be referred to as thein-plane indices.

One method of creating a uniaxial birefringent system is to biaxiallystretch (e.g., stretch along two dimensions) a multilayer stack in whichat least one of the materials in the stack has its index of refractionaffected by the stretching process (e.g., the index either increases ordecreases). Biaxial stretching of the multilayer stack may result indifferences between refractive indices of adjoining layers for planesparallel to both axes thus resulting in reflection of light in bothplanes of polarization.

A uniaxial birefringent material can have either positive or negativeuniaxial birefringence. Positive uniaxial birefringence occurs when thez-index is greater than the in-plane indices (nz>nx and ny). Negativeuniaxial birefringence occurs when the z-index is less than the in-planeindices (nz<nx and ny).

A biaxial birefringent material is defined as one in which the indicesof refraction in all three axes are different, e.g., nx≠ny≠nz. Again,the nx and ny indices will be referred to as the in-plane indices. Abiaxial birefringent system can be made by stretching the multilayerstack in one direction. In other words the stack is uniaxiallystretched. For purposes of the present discussion, the x direction willbe referred to as the stretch direction for biaxial birefringent stacks.

Uniaxial Birefringent Systems (Mirrors)

The optical properties and design considerations of uniaxialbirefringent systems will now be discussed. As discussed above, thegeneral conditions for a uniaxial birefringent material are nx=ny≠nz.Thus if each layer 102 and 104 in FIG. 10 is uniaxially birefringent,n1x=n1y and n2x=n2y. For purposes of the present discussion, assume thatlayer 102 has larger in-plane indices than layer 104, and that thusn1>n2 in both the x and y directions. The optical behavior of a uniaxialbirefringent multilayer system can be adjusted by varying the values ofn1z and n2z to introduce different levels of positive or negativebirefringence. The relationship between the various indices ofrefraction can be measured directly, or, the general relationship may beindirectly observed by analysis of the spectra of the resulting film asdescribed herein.

In the case of mirrors, the desired average transmission for light ofeach polarization and plane of incidence generally depends upon theintended use of the mirror. The average transmission along each stretchdirection at normal incidence for a narrow bandwidth mirror across a 100nm bandwidth within the visible spectrum is desirably less than 30%,preferably less than 20% and more preferably less than 10%. A desirableaverage transmission along each stretch direction at normal incidencefor a partial mirror ranges anywhere from, for example, 10% to 50%, andcan cover a bandwidth of anywhere between, for example, 100 nm and 450nm, depending upon the particular application. For a high efficiencymirror, average transmission along each stretch direction at normalincidence over the visible spectrum (400-700 nm) is desirably less than10%, preferably less than 5%, more preferably less than 2%, and evenmore preferably less than 1%. In addition, asymmetric mirrors may bedesirable for certain application. In that case, average transmissionalong one stretch direction may be desirably less than, for example,50%, while the average transmission along the other stretch directionmay be desirably less than, for example 20%, over a bandwidth of, forexample, the visible spectrum (400-700 nm), or over the visible spectrumand into the near infrared (e.g, 400-850 nm).

Equation 1 described above can be used to determine the reflectivity ofa single interface in a uniaxial birefringent system composed of twolayers such as that shown in FIG. 10. Equation 2, for s polarized light,is identical to that of the case of isotropic films (nx=ny=nz), so onlyequation 1 need be examined. For purposes of illustration, somespecific, although generic, values for the film indices will beassigned. Let n1x=n1y=1.75, n1z variable, n2x=n2y=1.50, andn2z=variable. In order to illustrate various possible Brewster angles inthis system, no=1.60 for the surrounding isotropic media.

FIG. 11 shows reflectivity versus angle curves for p-polarized lightincident from the isotropic medium to the birefringent layers, for caseswhere n1z is numerically greater than or equal to n2z (n1z≧n2z). Thecurves shown in FIG. 11 are for the following z-index values: a)n1z=1.75, n2z=1.50: b) n1z=1.75, n2z=1.57; c) n1z=1.70, n2z=1.60; d)n1z=1.65, n2z=1.60; e) n1z=1.61, n2z=1.60; and f) n1z=1.60=n2z. As n1zapproaches n2z, the Brewster angle, the angle at which reflectivity goesto zero, increases. Curves a-e are strongly angular dependent. However,when n1z=n2z (curve f), there is no angular dependence to reflectivity.In other words, the reflectivity for curve f is constant for all anglesof incidence. At that point, equation 1 reduces to the angularindependent form: (n2o−n1o)/(n2o+n1o). When n1z=n2z, there is noBrewster effect and there is constant reflectivity for all angles ofincidence.

FIG. 12 shows reflectivity versus angle of incidence curves for caseswhere n1z is numerically less than or equal to n2z. Light is incidentfrom isotropic medium to the birefringent layers. For these cases, thereflectivity monotonically increases with angle of incidence. This isthe behavior that would be observed for s-polarized light. Curve a inFIG. 12 shows the single case for s polarized light. Curves b-e showcases for p polarized light for various values of nz, in the followingorder: b) n1z=1.50, n2z=1.60: c) n1z=1.55, n2z=1.60:d) n1z=1.59,n2z=1.60; and e) n1z=1.60=n2z. Again, when n1z=n2z (curve e), there isno Brewster effect, and there is constant reflectivity for all angles ofincidence.

FIG. 13 shows the same cases as FIGS. 11 and 12 but for an incidentmedium of index no=1.0 (air). The curves in FIG. 13 are plotted for ppolarized light at a single interface of a positive uniaxial material ofindices n2x=n2y=1.50, n2z=1.60, and a negative uniaxially birefringentmaterial with n1x=n1y=1.75, and values of n1z, in the following order,from top to bottom, of: a) 1.50: b) 1.55; c) 1.59; d) 1.60; f) 1.61; g)1.65: h) 1.70: and i) 1.75. Again, as was shown in FIGS. 11 and 12, whenthe values of n1z and n2z match (curve d), there is no angulardependence to reflectivity.

FIGS. 11, 12 and 13 show that the cross-over from one type of behaviorto another occurs when the z-axis index of one film equals the z-axisindex of the other film. This is true for several combinations ofnegative and positive uniaxially birefringent, and isotropic materials.Other situations occur in which the Brewster angle is shifted to largeror smaller angles.

Various possible relationships between in-plane indices and z-axisindices are illustrated in FIGS. 14, 15 and 16. The vertical axesindicate relative values of indices and the horizontal axes are used toseparate the various conditions. Each Figure begins at the left with twoisotropic films, where the z-index equals the in-plane indices. As oneproceeds to the right, the in-plane indices are held constant and thevarious z-axis indices increase or decrease, indicating the relativeamount of positive or negative birefringence.

The case described above with respect to FIGS. 11, 12, and 13 isillustrated in FIG. 14. The in-plane indices of material one are greaterthan the in-plane indices of material two, material 1 has negativebirefringence (n1z less than in-plane indices), and material two haspositive birefringence (n2z greater than in-plane indices). The point atwhich the Brewster angle disappears and reflectivity is constant for allangles of incidence is where the two z-axis indices are equal. Thispoint corresponds to curve f in FIG. 11, curve e in FIG. 12 or curve din FIG. 13.

In FIG. 15, material one has higher in-plane indices than material two,but material one has positive birefringence and material two hasnegative birefringence. In this case, the Brewster minimum can onlyshift to lower values of angle.

Both FIGS. 14 and 15 are valid for the limiting cases where one of thetwo films is isotropic. The two cases are where material one isisotropic and material two has positive birefringence, or material twois isotropic and material one has negative birefringence. The point atwhich there is no Brewster effect is where the z-axis index of thebirefringent material equals the index of the isotropic film.

Another case is where both films are of the same type, i.e., bothnegative or both positive birefringent. FIG. 16 shows the case whereboth films have negative birefringence. However, it shall be understoodthat the case of two positive birefringent layers is analogous to thecase of two negative birefringent layers shown in FIG. 16. As before,the Brewster minimum is eliminated only if one z-axis index equals orcrosses that of the other film.

Yet another case occurs where the in-plane indices of the two materialsare equal, but the z-axis indices differ. In this case, which is asubset of all three cases shown in FIGS. 14-16, no reflection occurs fors polarized light at any angle, and the reflectivity for p polarizedlight increases monotonically with increasing angle of incidence. Thistype of article has increasing reflectivity for p-polarized light asangle of incidence increases, and is transparent to s-polarized light.This article can be referred to as a “p-polarizer”.

The above described principles and design considerations describing thebehavior of uniaxially birefringent systems can be applied to createmultilayer stacks having the desired optical effects for a wide varietyof circumstances and applications. The indices of refraction of thelayers in the multilayer stack can be manipulated and tailored toproduce devices having the desired optical properties. Many negative andpositive uniaxial birefringent systems can be created with a variety ofin-plane and z-axis indices, and many useful devices can be designed andfabricated using the principles described here.

Biaxial Birefringent Systems (Polarizers)

Referring again to FIG. 10, two component orthogonal biaxialbirefringent systems and the design considerations affecting theresultant optical properties will now be described. Again, the systemcan have many layers, but an understanding of the optical behavior ofthe stack is achieved by examining the optical behavior at oneinterface.

A biaxial birefringent system can be designed to give high reflectivityfor light with its plane of polarization parallel to one axis, for abroad range of angles of incidence, and simultaneously have lowreflectivity and high transmission for light with its plane ofpolarization parallel to the other axis for a broad range of angles ofincidence. As a result, the biaxial birefringent system acts as apolarizer, transmitting light of one polarization and reflecting lightof the other polarization. By controlling the three indices ofrefraction of each film, nx, ny and nz, the desired polarizer behaviorcan be obtained. Again, the indices of refraction can be measureddirectly or can be indirectly observed by analysis of the spectra of theresulting film, as described herein.

Referring again to FIG. 10, the following values to the film indices areassigned for purposes of illustration: n1x=1.88, n1y=1.64, n1z=variable,n2x=1.65, n2y=variable, and n2z=variable. The x direction is referred toas the extinction direction and the y direction as the transmissiondirection.

Equation 1 can be used to predict the angular behavior of the biaxialbirefringent system for two important cases of light with a plane ofincidence in either the stretch (xz plane) or the non-stretch (yz plane)directions. The polarizer is a mirror in one polarization direction anda window in the other direction. In the stretch direction, the largeindex differential of 1.88−1.65=0.23 in a multilayer stack with hundredsof layers will yield very high reflectivities for s-polarized light. Forp-polarized light the reflectance at various angles depends on then1z/n2z index differential.

In many applications, the ideal reflecting polarizer has highreflectance along one axis (the so-called extinction axis) and zeroreflectance along the other (the so-called transmission axis), at allangles of incidence. For the transmission axis of a polarizer, itgenerally desirable to maximize transmission of light polarized in thedirection of the transmission axis over the bandwidth of interest andalso over the range of angles of interest. Average transmission atnormal incidence for a narrow band polarizer across a 100 nm bandwidthis desirably at least 50%, preferably at least 70% and more preferablyat least 90%. The average transmission at 60 degrees from the normal forp-polarized light (measured along the transmission axis) for a narrowband polarizer across a 100 nm bandwidth is desirably at least 50%,preferably at least 70% and more preferably at least 80%.

The average transmission at normal incidence for a polarizer in thetransmission axis across the visible spectrum (400-700 nm for abandwidth of 300 nm) is desirably at least 50%, preferably at least 70%,more preferably at least 85%, and even more preferably at least 90%. Theaverage transmission at 60 degrees from the normal (measured along thetransmission axis) for a polarizer from 400-700 nm is desirably at least50%, preferably at least 70%, more preferably at least 80%, and evenmore preferably at least 90%.

For certain applications, high reflectivity in the transmission axis atoff-normal angles are preferred. The average reflectivity for lightpolarized along the transmission axis should be more than 20% at anangle of at least 20 degrees from the normal.

If some reflectivity occurs along the transmission axis, the efficiencyof the polarizer at off-normal angles may be reduced. If thereflectivity along the transmission axis is different for variouswavelengths, color may be introduced into the transmitted light. One wayto measure the color is to determine the root mean square (EMS) value ofthe transmission at a selected angle or angles over the wavelength rangeof interest. The % RMS color, ^(C) ^(_(RMS)) , can be determinedaccording to the equation:$C_{{RM}\quad S} = \frac{\int_{\lambda 1}^{\lambda 2}{\left( \left( {T - \overset{\_}{T}} \right)^{2} \right)^{1/2}{\lambda}}}{\overset{\_}{T}\left( {{\lambda 2} - {\lambda 1}} \right)}$

where the range λ1 to λ2 is the wavelength range, or bandwidth, ofinterest. T is the transmission along the transmission axis, and{overscore (T)} is the average transmission along the transmission axisin the wavelength range of interest.

For applications where a low color polarizer is desirable, the % RMScolor should be less than 10%, preferably less than 8%, more preferablyless than 3.5%, and even more preferably less than 2.1% at an angle ofat least 30 degrees from the normal, preferably at least 45 degrees fromthe normal, and even more preferably at least 60 degrees from thenormal.

Preferably, a reflective polarizer combines the desired % RMS coloralong the transmission axis for the particular application with thedesired amount of reflectivity along the extinction axis across thebandwidth of interest. For example, for narrow band polarizers having abandwidth of approximately 100 nm, average transmission along theextinction axis at normal incidence is desirably less than 50%,preferably less than 30%, more preferably less than 10%, and even morepreferably less than 3%. For polarizers having a bandwidth in therisible range (400-700 nm, or a bandwidth of 300 nm), averagetransmission along the extinction axis at normal incidence is desirablyless than 40%, more desirably less than 25%, preferably less than 15%,more preferably less than 5% and even more preferably less than 3%.

Reflectivity at off-normal angles, for light with its plane ofpolarization parallel to the transmission axis may be caused by a largez-index mismatch, even if the in-plane y indices are matched. Theresulting system thus has large reflectivity for p, and is highlytransparent to s polarized light. This case was referred to above in theanalysis of the mirror cases as a “p polarizer”.

For uniaxially stretched polarizers, performance depends upon therelationships between the alternating layer indices for all three (x, y,and z) directions. As described herein, it is desirable to minimize they and z index differentials for a high efficiency polarizer.Introduction of a y-index mismatch is describe to compensate for az-index mismatch. Whether intentionally added or naturally occurring,any index mismatch will introduce some reflectivity. An important factorthus is making the x-index differential larger than the y-and z-indexdifferentials. Since reflectivity increases rapidly as a function ofindex differential in both the stretch and non-stretch directions, theratios Δny/Δnx and Δnz/Δnx should be minimized to obtain a polarizerhaving high extinction along one axis across the bandwidth of interestand also over a broad range of angles, while preserving hightransmission along the orthogonal axis. Ratios of less than 0.05, 0.1 or0.25 are acceptable. Ideally, the ratio Δnz/Δnx is 0, but ratios of lessthan 0.25 or 0.5 also produce a useable polarizer.

FIG. 17 shows the reflectivity (plotted as-Log[1-R]) at 75° for ppolarized light with its plane of incidence in the non-stretchdirection, for an 800 layer stack of PEN/coPEN. The reflectivity isplotted as function of wavelength across the visible spectrum (400-700nm). The relevant indices for curve a at 550 nm are n1y=1.64, n1z=1.52,n2y=1.64 and n2z=1.63. The model stack design is a linear thicknessgrade for quarterwave pairs, where each pair thickness is given bydn=do+do(0.003)n. All layers were assigned a random thickness error witha gaussian distribution and a 5% standard deviation.

Curve a shows high off-axis reflectivity across the visible spectrumalong the transmission axis (the y-axis) and that different wavelengthsexperience different levels of reflectivity. This is due to the largez-index mismatch (Δnz=0.11). Since the spectrum is sensitive to layerthickness errors and spatial nonuniformities, such as film caliper, thisgives a biaxial birefringent system with a very nonuniform and“colorful” appearance. Although a high degree of color may be desirablefor certain applications, it is desirable to control the degree ofoff-axis color, and minimize it for those applications requiring auniform, low color appearance, such as liquid crystal displays or othertypes of displays.

Off-axis reflectivity, and off-axis color can be minimized byintroducing an index mismatch to the non-stretch in-plane indices (n1yand n2y) that create a Brewster condition off axis, while keeping thes-polarization reflectivity to a minimum.

FIG. 18 explores the effect of introducing a y-index mismatch inreducing off-axis reflectivity along the transmission axis of a biaxialbirefringent system. With n1z=1.52 and n2z=1.63 (Δnz=0.11), thefollowing conditions are plotted for p polarized light: a) n1y=n2y=1.64:b) n1y=1.64, n2y=1.62; c) n1y=1.64, n2y=1.66. Curve a shows thereflectivity where the in-plane indices n1y and n2y are equal. Curve ahas a reflectance minimum at 0°, but rises steeply after 20°. For curveb, n1y>n2y, and reflectivity increases rapidly. Curve c, where n1y<n2y,has a reflectance minimum at 38°, but rises steeply thereafter.Considerable reflection occurs as well for s polarized light forn1y≠n2y, as shown by curve d. Curves a-d of FIG. 18 indicate that thesign of the y-index mismatch (n1y-n2y) should be the same as the z-indexmismatch (n1z-n2z) for a Brewster minimum to exist. For the case ofn1y=n2y, reflectivity for s polarized light is zero at all angles.

By reducing the z-axis index difference between layers, the off axisreflectivity can be further reduced. If n1z is equal to n2z. FIG. 13indicates that the extinction axis will still have a high reflectivityoff-angle as it does at normal incidence, and no reflection would occuralong the nonstretch axis at any angle because both indices are matched(e.g.n1y=n2y and n1z=n2z).

Exact matching of the two y indices and the two z indices may not bepossible in some multilayer systems. If the z-axis indices are notmatched in a polarizer construction, introduction of a slight mismatchmay be desired for in-plane indices n1y and n2y. This can be done byblending additional components into one or both of the material layersin order to increase or decrease the respective v index as describedbelow in Example 15. Blending a second resin into either the polymerthat forms the highly birefringent layers or into the polymer that formsthe selected polymer layers may be done to modify reflection for thetransmission axis at normal and off-normal angles, or to modify theextinction of the polarizer for light polarized in the extinction axis.The second, blended resin may accomplish this by modifying thecrystallinity and the index of refraction of the polymer layers afterorientation.

Another example is plotted in FIG. 19, assuming n1z=1.56 and n2z=1.60(Δnz=0.04), with the following y indices a) n1y=1.64, n2y=1.65; b)n1y=1.64, n2y=1.63. Curve c is for s-polarized light for either case.Curve a, where the sign of the v-index mismatch is the same as thez-index mismatch, results in the lowest off-angle reflectivity.

The computed off-axis reflectance of an 800 layer stack of films at 75°angle of incidence with the conditions of curve a in FIG. 19 is plottedas curve b in FIG. 17. Comparison of curve b with curve a in FIG. 17shows that there is far less off-axis reflectivity, and therefore lowerperceived color and better uniformity, for the conditions plotted incurve b. The relevant indices for curve b at 550 nm are n1y=1.64,n1z=1.56, n2y=1.65 and n2z=1.60.

FIG. 20 shows a contour plot of equation 1 which summarizes the off axisreflectivity discussed in relation to FIG. 10 for p-polarized light. Thefour independent indices involved in the non-stretch direction have beenreduced to two index mismatches, Δnz and Δny. The plot is an average of6 plots at various angles of incidence from 0° to 75° in 15 degreeincrements. The reflectivity ranges from 0.4×10−4 for contour j, to4.0×10−4 for contour a, in constant increments of 0.4×10−4. The plotsindicate how high reflectivity caused by an index mismatch along oneoptic axis can be offset by a mismatch along the other axis.

Thus, by reducing the z-index mismatch between layers of a biaxialbirefringent systems, and/or by introducing a y-index mismatch toproduce a Brewster effect, off-axis reflectivity, and therefore off-axiscolor, are minimized along the transmission axis of a multilayerreflecting polarizer.

It should also be noted that narrow band polarizers operating over anarrow wavelength range can also be designed using the principlesdescribed herein. These can be made to produce polarizers in the red,green, blue, cyan, magenta, or yellow bands, for example.

An ideal reflecting polarizer should transmit all light of onepolarization, and reflect all light of the other polarization. Unlesslaminated on both sides to glass or to another film with a clear opticaladhesive, surface reflections at the air/reflecting polarizer interfacewill reduce the transmission of light of the desired polarization. Thus,it may in some cases be useful to add an antireflection (AR) coating tothe reflecting polarizer. The AR coating is preferably designed todereflect a film of index 1.64 for PEN based polarizers in air, becausethat is the index of all layers in the nonstretch (y) direction. Thesame coating will have essentially no effect on the stretch directionbecause the alternating index stack of the stretch direction has a veryhigh reflection coefficient irrespective of the presence or absence ofsurface reflections. Any AR coating known in the art could be applied,provided that the coating does not overheat or damage the multilayerfilm being coated. An exemplary coating would be a quarterwave thickcoating of low index material, ideally with index near the square rootof 1.64 (for PEN based materials).

Materials Selection and Processing

With the above-described design considerations established, one ofordinary skill will readily appreciate that a wide variety of materialscan be used to form multilayer mirrors or polarizers according to theinvention when processed under conditions selected to yield the desiredrefractive index relationships. The desired refractive indexrelationships can be achieved in a variety of ways, including stretchingduring or after film formation (e.g., in the case of organic polymers),extruding (e.g., in the case of liquid crystalline materials), orcoating. In addition, it is preferred that the two materials havesimilar rheological properties (e.g., melt viscosities) such that theycan be co-extruded.

In general, appropriate combinations may be achieved by selecting, asthe first material, a crystalline or semi-crystalline material,preferably a polymer. The second material, in turn, may be crystalline,semi-crystalline, or amorphous. The second material may have abirefringence opposite to or the same as that of the first material. Or,the second material may have no birefringence.

Specific examples of suitable materials include polyethylene naphthalate(PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,51-, 2,7- , and 2,3-PEN),polyalkylene terephthalates (e.g., polyethylene terephthalate,polybutylene terephthalate, and poly-1,4-cyclohexanedimethyleneterephthalate), polyimides (e.g., polyacrylic imides), polyetherimides,atactic polystyrene, polycarbonates, polymethacrylates (e.g.,polyisobutyl methacrylate, polypropylmethacrylate,polyethylmethacrylate, and polymethylmethacrylate), polyacrylates (e.g.,polybutylacrylate and polymethylacrylate), syndiotactic polystyrene(sPS), syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene), fluorinatedpolymers (e.g., perfluoroalkoxy resins, polytetrafluoroethylene,fluorinated ethylene-propylene copolymers, polyvinylidene fluoride, andpolychlorotrifluoroethylene), chlorinated polymers (e.g., polyvinylidenechloride and polyvinylchloride), polysulfones, polyethersulfones,polyacrylonitrile, polyamides, silicone resins, epoxy resins,polyvinylacetate, polyether-amides, ionomers resins, elastomers (e.g.,polybutadiene, polyisoprene, and neoprene), and polyurethanes. Alsosuitable are copolymers, e.g., copolymers of PEN (e.g., copolymers of2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, oresters thereof, with (a) terephthalic acid, or esters thereof: (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof: (d) alkane glycols: (e) cycloalkane glycols (e.g., cyclohexanedimethanol diol): (f) alkane dicarboxylic acids: and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), copolymers ofpolyalkylene terephthalates (e.g., copolymers of terephthalic acid, oresters thereof, with (a) naphthalene dicarboxylic acid, or estersthereof: (b) isophthalic acid, or esters thereof, (c) phthalic acid, oresters thereof, (d) alkane glycols: (e) cycloalkane glycols (e.g.,cyclohexane dimethanol diol): (f) alkane dicarboxylic acids; and/or (g)cycloalkane dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),and styrene copolymers (e.g., styrene-butadiene copolymers andstyrene-acrylonitrile copolymers) 4,4′-bibenzoic acid and ethyleneglycol. In addition, each individual layer may include blends of two ormore of the above-described polymers or copolymers (e.g., blends of SPSand atactic polystyrene). The coPEN described may also be a blend ofpellets where at least one component is a polymer based on naphthalenedicarboxylic acid and other components are other polyesters orpolycarbonates, such as a PET, a PEN or a co-PEN.

Particularly preferred combinations of layers in the case of polarizersinclude PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/sPS,PET/sPS, PEN/Eastar, and PET/Eastar, where “co-PEN” refers to acopolymer or blend based upon naphthalene dicarboxylic acid (asdescribed above) and Eastar is polycyclohexanedimethylene terephthalatecommercially available from Eastman Chemical Co.

Particularly preferred combinations of layers in the case of mirrorsinclude PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV, PEN/co-PET, and PET/sPS,where “co-PET” refers to a copolymer or blend based upon terephthalicacid (as described above), Ecdel is a thermoplastic polyestercommercially available from Eastman Chemical Co. and THV is afluoropolymer commercially available from 3M Co.

The number of layers in the device is selected to achieve the desiredoptical properties using the minimum number of layers for reasons offilm thickness, flexibility and economy. In the case of both polarizersand mirrors, the number of layers is preferably less than 10.000, morepreferably less than 5.000, and (even more preferably) less than 2.000.

As discussed above, the ability to achieve the desired relationshipsamong the various indices of refraction (and thus the optical propertiesof the multilayer device) is influenced by the processing conditionsused to prepare the multilayer device. In the case of organic polymerswhich can be oriented by stretching, the devices are generally preparedby co-extruding the individual polymers to form a multilayer film andthen orienting the film by stretching at a selected temperature,optionally followed by heat-setting at a selected temperature.Alternatively, the extrusion and orientation steps may be performedsimultaneously. In the case of polarizers, the film is stretchedsubstantially in one direction (uniaxial orientation), while in the caseof mirrors the film is stretched substantially in two directions(biaxial orientation).

The film may be allowed to dimensionally relax in the cross-stretchdirection from the natural reduction in cross-stretch (equal to thesquare root of the stretch ratio) to being constrained (i.e., nosubstantial change in cross-stretch dimensions). The film may bestretched in the machine direction, as with a length orienter, in widthusing a tenter.

The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, andcross-stretch relaxation are selected to yield a multilayer devicehaving the desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled with, e.g. a relatively low stretch temperature. Itwill be apparent to one of ordinary skill how to select the appropriatecombination of these variables to achieve the desired multilayer device.In general, however, a stretch ratios in the range from 1:2 to 1:10(more preferably 1:3 to 1:7) in the stretch direction and from 1:0.5 to1:10 (more preferably from 1:0.5 to 1:7) orthogonal to the stretchdirection is preferred.

Suitable multilayer devices may also be prepared using techniques suchas spin coating (e.g. as described in Boese et al. J. Polym. Sci.: PartB. 30:13231 (1992) for birefringent polyimides, and vacuum deposition(e.g. as described by Zang et. al. Appl. Phys. Letters, 59:823 (1991)for crystalline organic compounds: the latter technique is particularlyuseful for certain combinations of crystalline organic compounds andinorganic materials.

The invention will now be described by way of the following examples. Inthe examples, because optical absorption is negligible, reflectionequals 1 minus transmission (R=1−T).

EXAMPLE 1 (Polarizer)

PEN and a 70 naphthalate/30 terephthalate copolyester (coPEN) weresynthesized in a standard polyester resin kettle using ethylene glycolas the diol. The intrinsic viscosity of both the PEN and the coPEN wasapproximately 0.6 dl/g. Single layer films of PEN and coPEN wereextruded and then uniaxially stretched, with the sides restrained, atapproximately 150° C. As extruded, the PEN exhibited an isotropicrefractive index of about 1.65, and the coPEN was characterized by anisotropic refractive index of about 1.64. By isotropic is meant that therefractive indices associated with all axes in the plane of the film aresubstantially equal. Both refractive index values were observed at 550nm. After stretching at a 5:1 stretch ratio, the refractive index of thePEN associated with the oriented axis increased to approximately 1.88.The refractive index associated with the transverse axis droppedslightly to 1.64. The refractive index of the coPEN film afterstretching at a 5:1 stretch ratio remained isotropic at approximately1.64.

A satisfactory multilayer polarizer was then made of alternating layersof PEN and coPEN by coextrusion using a 51-slot feed block which fed astandard extrusion die. The extrusion was run at approximately 295° C.The PEN was extruded at approximately 23 lb/hr and the coPEN wasextruded at approximately 22.3 lb,hr. The PEN skin layers wereapproximately three times as thick as the layers within the extrudedfilm stack. All internal layers were designed to have an optical ¼wavelength thickness for light of about 1300 nm. The 51-layer stack wasextruded and cast to a thickness of approximately 0.0029 inches, andthen uniaxially stretched with the sides restrained at approximately a5:1 stretch ratio at approximately 150° C. The stretched film had athickness of approximately 0.0005 inches.

The stretched film was then heat set for 30 seconds at approximately230° C. in an air oven. The optical spectra were essentially the samefor film that was stretched and for film that was subsequently heat set.

FIG. 5 is a graphical view of percent measured transmission of the51-layer stack in both an oriented direction 50 and in a transversedirection 52 prior to heat setting.

Eight 51-layered polarizers, each made as described above, were combinedusing a fluid to eliminate the air gaps forming a polarizer of 408optical layers. FIG. 6 is a graph that characterizes the 408 layersshowing percent transmission from 350 to 1,800 nm in both an orienteddirection 54 and in a transverse direction 56.

EXAMPLE 2 (Polarizer)

A satisfactory 204layered polarizer was made by extruding PEN and coPENin the 51-slot feedblock as described in Example 1 and then employingtwo layer doubling multipliers in series in the extrusion. Themultipliers divide the extruded material exiting the feed block into twohalf-width flow streams, then stack the hall-width flow streams on topof each other. U.S. Pat. No. 3,565,985 describes similar coextrusionmultipliers. The extrusion was performed at approximately 295° C. usingPEN at an intrinsic viscosity of 0.50 dl/g at 22.5 lb/hr while the coPENat an intrinsic viscosity of 0.60 dl/g was run at 16.5 lb hr. The castweb was approximately 0.0038 inches in thickness and was uniaxiallystretched at a 5:1 ratio in a longitudinal direction with the sidesrestrained at an air temperature of 140° C. during stretching. Exceptfor skin layers, all pairs of layers were designed to be ½ wavelengthoptical thickness for 550 nm light. In the transmission spectra of FIG.7 two reflection peaks in the oriented direction 60 are evident from thetransmission spectra, centered about 550 nm. The double peak is mostlikely a result of film errors introduced in the layer multipliers, andthe broad background a result of cumulative film errors throughout theextrusion and casting process. The transmission spectra in thetransverse direction is indicated by 58. Optical extinction of thepolarizer can be greatly improved by laminating two of these filmstogether with an optical adhesive.

Two 204-layer polarizers made as described above were thenhand-laminated using an optical adhesive to produce a 408-layered filmstack. Preferably the refractive index of the adhesive should match theindex of the isotropic coPEN layer. The reflection peaks evident in FIG.7 are smoothed out for a laminated sample, as shown in FIG. 8. Thisoccurs because the peak reflectivity occurs at different wavelengths fordifferent areas of the film, in a random pattern. This effect is oftenreferred to as “iridescence”. Lamination of two films reducesiridescence because the random variations in color do not match from onefilm to another, and tend to cancel when the films are overlapped.

FIG. 8 illustrates the transmission data in both the oriented direction64 and transverse direction 62. Over 80 percent of the light in oneplane of polarization is reflected for wavelengths in a range fromapproximately 450 to 650 nm.

The iridescence is essentially a measure of nonuniformities in the filmlayers in one area versus adjacent areas. With perfect thicknesscontrol, a film stack centered at one wavelength would have no colorvariation across the sample. Multiple stacks designed to reflect theentire visible spectrum will have iridescence if significant light leaksthrough random areas at random wavelengths, due to layer thicknesserrors. The large differential index between film layers of the polymersystems presented here enable film reflectivities of greater than 99percent with a modest number of layers. This is a great advantage ineliminating iridescence if proper layer thickness control can beachieved in the extrusion process. Computer based optical modeling hasshown that greater than 99 percent reflectivity across most of thevisible spectrum is possible with only 600 layers for a PEN/coPENpolarizer if the layer thickness values are controlled with a standarddeviation of less than or equal to 10 percent.

EXAMPLE 3 (PET:Ecdel, 601, Mirror)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dog (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rateof 75 pounds per hour and Ecdel 9966 (a thermoplastic elastomeravailable from Eastman Chemical) was delivered by another extruder at arate of 65 pounds per hour. The PET was on the skin layers. Thefeedblock method (such as that described in U.S. Pat. No. 3,801,429) wasused to generate 151 layers which was passed through two multipliersproducing an extrudate of 601 layers. U.S. Pat. No. 3.565.985 describesexemplary coextrusion multipliers. The web was length oriented to a drawratio of about 3.6 with the web temperature at about 210° F. The filmwas subsequently preheated to about 235° F. in about 50 seconds anddrawn in the transverse direction to a draw ratio of about 4.0 at a rateof about 6% per second. The film was then relaxed about 5% of itsmaximum width in a heat-set oven set at 400° F. The finished filmthickness was 2.5 mil.

The cast web produced was rough in texture on the air side, and providedthe transmission as shown in FIG. 21. The % transmission for p-polarizedlight at a 60° angle (curve b) is similar the value at normal incidence(curve a) (with a wavelength shift).

For comparison, film made by Mearl Corporation, presumably of isotropicmaterials (see FIG. 22) shows a noticeable loss in reflectivity forp-polarized light at a 60° angle (curve b, compared to curve a fornormal incidence).

EXAMPLE 4 (PET:Ecdel, 151, Mirror)

A coextruded film containing 151 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wtphenol/40 wt. % dichlorobenzene) was delivered by one extruder at a rateof 75 pounds per hour and Ecdel 9966 (a thermoplastic elastomeravailable from Eastman Chemical) was delivered by another extruder at arate of 65 pounds per hour. The PET was on the skin layers. Thefeedblock method was used to generate 151 layers. The web was lengthoriented to a draw ratio of about 3.5 with the web temperature at about210° F. The film was subsequently preheated to about 215° F. in about 12seconds and drawn in the transverse direction to a draw ratio of about4.0 at a rate of about 25% per second. The film was then relaxed about5% of its maximum width in a heat-set oven set at 400° F. in about 6seconds. The finished film thickness was about 0.6 mil.

The transmission of this film is shown in FIG. 23. The % transmissionfor p-polarized light at a 60° angle (curve b) is similar the value atnormal incidence (curve a) with a wavelength shift. At the sameextrusion conditions the web speed was slowed down to make an infraredreflecting film with a thickness of about 0.8 mils. The transmission isshown in FIG. 24 (curve a at normal incidence, curve b at 60 degrees).

EXAMPLE 5 (PEN:Ecdel, 225, Mirror)

A coextruded film containing 225 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 18 poundsper hour and Ecdel 9966 (a thermoplastic elastomer available fromEastman Chemical) was delivered by another extruder at a rate of 17pounds per hour. The PEN was on the skin layers. The feedblock methodwas used to generate 57 layers which was passed through two multipliersproducing an extrudate of 225 layers. The cast web was 12 mils thick and12 inches wide. The web was later biaxially oriented using a laboratorystretching device that uses a pantograph to grip a square section offilm and simultaneously stretch it in both directions at a uniform rate.A 7.46 cm square of web was loaded into the stretcher at about 100° C.and heated to 130° C. in 60 seconds. Stretching then commenced at100%/sec (based on original dimensions) until the sample was stretchedto about 3.5×3.5. Immediately after the stretching the sample was cooledby blowing room temperature air on it.

FIG. 25 shows the optical response of this multilayer film (curve a atnormal incidence, curve b at 60 degrees). Note that the % transmissionfor p-polarized light at a 60° angle is similar to what it is at normalincidence (with some wavelength shift).

EXAMPLE 6 (PEN:THV 500, 449, Mirror)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 56 poundsper hour and THV 500 (a fluoropolymer available from Minnesota Miningand Manufacturing Company) was delivered by another extruder at a rateof 11 pounds per hour. The PEN was on the skin layers and 50% of the PENwas present in the two skin layers. The feedblock method was used togenerate 57 layers which was passed through three multipliers producingan extrudate of 449 layers. The cast web was 20 mils thick and 12 incheswide. The web was later biaxially oriented using a laboratory stretchingdevice that uses a pantograph to grip a square section of film andsimultaneously stretch it in both directions at a uniform rate. A 7.46cm square of web was loaded into the stretcher at about 100° C. andheated to 140° C. in 60 seconds. Stretching then commenced at 10%/sec(based on original dimensions) until the sample was stretched to about3.5×3.5. Immediately after the stretching the sample was cooled byblowing room temperature air at it.

FIG. 26 shows the transmission of this multilayer film, Again, curve ashows the response at normal incidence, while curve b shows the responseat 60 degrees.

EXAMPLE 7 (PEN:CoPEN, 449-Low Color Polarizer)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.56 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 43 poundsper hour and a coPEN (70 mol % 2,6 NDC and 30 mol % DMT) with anintrinsic viscosity of 0.52 (60 wt. % phenol/40 wt. % dichlorobenzene)was delivered by another extruder at a rate of 25 pounds per hour. ThePEN was on the skin layers and 40% of the PEN was present in the twoskin layers. The feedblock method was used to generate 57 layers whichwas passed through three multipliers producing an extrudate of 449layers. The cast web was 10 mils thick and 12 inches wide. The web waslater uniaxially oriented using a laboratory stretching device that usesa pantograph to grip a square section of film and stretch it in onedirection while it is constrained in the other at a uniform rate. A 7.46cm square of web was loaded into the stretcher at about 100° C. andheated to 140° C. in 60 seconds. Stretching then commenced at 10%/sec(based on original dimensions) until the sample was stretched to about5.5×1. Immediately after the stretching the sample was cooled by blowingroom temperature air at it.

FIG. 27 shows the transmission of this multilayer film. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the very high transmissionof light polarized in the non-stretch direction at both normal and 60°incidence. Average transmission for curve a over 400-700 nm is 87.1%,while average transmission for curve b over 400-700 nm is 97.1%.Transmission is higher for p-polarized light at 60° incidence becausethe air/PEN interface has a Brewster angle near 60°, so the transmissionat 60° incidence is nearly 100%. Also note the high extinction of lightpolarized in the stretched direction in the visible range (400-700nm)shown by curve c, where the average transmission is 21.0%. The % RMScolor for curve a is 1.5%. The % RMS color for curve b is 1.4%.

EXAMPLE 8 (PEN:CoPEN. 601-High Color Polarizer)

A coextruded film containing 601 layers was produced by extruding theweb and two days later orienting the film on a different tenter thandescribed in all the other examples. A Polyethylene Naphthalate (PEN)with an Intrinsic Viscosity of 0.5 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 75 poundsper hour and a coPEN (70 mol % 2,6 NDC and 30 mol % DMT) with an IV of0.55 dl/g (60 wt. % phenol/40 wt. 7 dichlorobenzene) was delivered byanother extruder at a rate of 65 pounds per hour. The PEN was on theskin layers. The feedblock method was used to generate 151 layers whichwas passed through two multipliers producing an extrudate of 601 layers.U.S. Pat. No. 3.565.985 describes similar coextrusion multipliers. Allstretching was done in the tenter. The film was preheated to about 280°F. in about 20 seconds and drawn in the transverse direction to a drawratio of about 4.4 at a rate of about 6% per second. The film was thenrelaxed about 2% of its maximum width in a heat-set oven set at 460° F.The finished film thickness was 1.8 mil.

The transmission of the film is shown in FIG. 28. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the nonuniform transmissionof p-polarized light at both normal and 60° incidence. The averagetransmission for curve a over 400-700 nm is 84.1%, while the averagetransmission for curve b over 400-700 nm is 68.2%. The averagetransmission for curve c is 9.1%. The % RMS color for curve a is 1.4%,and the % RMS color for curve b is 11.2%.

EXAMPLE 9 (PET:CoPEN, 449, Polarizer)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A Polyethylene Terephthalate (PET) with anIntrinsic Viscosity of 0.60 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 26 poundsper hour and a coPEN (70 mol % 2,6 NDC and 30 mol % DMT) with anintrinsic viscosity of 0.53 (60 wt. % phenol/40 wt. % dichlorobenzene)was delivered by another extruder at a rate of 24 pounds per hour. ThePET was on the skin layers. The feedblock method was used to generate 57layers which was passed through three multipliers producing an extrudateof 449 layers. U.S. Pat. No. 3.565,985 describes similar coextrusionmultipliers. The cast web was 7.5 mils thick and 12 inches wide. The webwas later uniaxially oriented using a laboratory stretching device thatuses a pantograph to grip a square section of film and stretch it in onedirection while it is constrained in the other at a uniform rate. A 7.46cm square of web was loaded into the stretcher at about 100° C. andheated to 120° C. in 60 seconds. Stretching then commenced at 10%/sec(based on original dimensions) until the sample was stretched to about5.0×1. Immediately after the stretching the sample was cooled by blowingroom temperature air at it. The finished film thickness was about 1.4mil. This film had sufficient adhesion to survive the orientationprocess with no delamination.

FIG. 29 shows the transmission of this multilayer film. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence, Note the very high transmissionof p-polarized light at both normal and 60° incidence. The averagetransmission for curve a over 400-700 nm is 88.0%, and the averagetransmission for curve b over 400-700 nm is 91.2%. The averagetransmission for curve c over 400-700 mn is 27.9%. The % RMS color forcurve a is 1.4%, and the % RMS color for curve b is 4.8%.

EXAMPLE 10 (PEN7.CoPEN, 601, Polarizer)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt %Phenol plus 40 wt % dichlorobenzene) was delivered by on extruder at arate of 75 pounds per hour and the coPEN was delivered by anotherextruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole %2,6 naphthalene dicarboxylate methyl ester, 15% dimethyl isophthalateand 15% dimethyl terephthalate with ethylene glycol. The feedblockmethod was used to generate 151 layers. The feedblock was designed toproduce a gradient distribution of layers with a ration of thickness ofthe optical layers of 1.22 for the PEN and 1.22 for the coPEN. The PENskin layers were coextruded on the outside of the optical stack with atotal thickness of 8% of the coextruded layers. The optical stack wasmultiplied by two sequential multipliers. The nominal multiplicationratio of the multipliers were 1.2 and 1.27, respectively. The film wassubsequently preheated to 310° F. in about 40 seconds and drawn in thetransverse direction to a draw ratio of about 5.0 at a rate of 6% persecond. The finished film thickness was about 2 mils.

FIG. 30 shows the transmission for this multilayer film. Curve a showstransmission of light polarized in the non-stretch direction at normalincidence, curve b shows transmission of p-polarized light at 60°incidence, and curve c shows transmission of light polarized in thestretch direction at normal incidence. Note the very high transmissionof p-polarized light at both normal and 600° incidence (80-100%). Alsonote the very high extinction of light polarized in the stretcheddirection in the visible range (400-700 nm) shown by curve c. Extinctionis nearly 100% between 500 and 650 nm.

EXAMPLE 11 (PEN:sPS, 481, Polarizer)

A 481 layer multilayer film was made from a polyethylene naphthalate(PEN) with an intrinsic viscosity of 0.56 dl/g measured in 60 wt. %phenol and 40 wt % dichlorobenzene purchased from Eastman Chemicals anda syndiotactic polystyrene (sPS) homopolymer (weight average molecularweight=200.000 Daltons, sampled from Dow Corporation). The PEN was onthe outer layers and was extruded at 26 pounds per hour and the sPS at23 pounds per hour. The feedblock used produced 61 layers with each ofthe 61 being approximately the same thickness. After the feedblock three(2×) multipliers were used. Equal thickness skin layers containing thesame PEN fed to the feedblock were added after the final multiplier at atotal rate of 22 pounds per hour. The web was extruded through a 12″wide die to a thickness or about 0.011 inches (0.276 mm). The extrusiontemperature was 290° C.

This web was stored at ambient conditions for nine days and thenuniaxially oriented on a tenter. The film was preheated to about 320° F.(160° C.) in about 25 seconds and drawn in the transverse direction to adraw ratio of about 6:1 at a rate of about 28% per second. No relaxationwas allowed in the stretched direction. The finished film thickness wasabout 0.0018 inches (0.046 mm).

FIG. 31 shows the optical performance of this PEN:sPS reflectivepolarizer containing 481 layers. Curve a shows transmission of lightpolarized in the non-stretch direction at normal incidence, curve bshows transmission of p-polarized light at 60° incidence, and curve cshows transmission of light polarized in the stretch direction at normalincidence. Note the very high transmission of p-polarized light at bothnormal and 60° incidence. Average transmission for curve a over 400-700nm is 86.2%, the average transmission for curve b over 400-700 nm is79.7%. Also note the very high extinction of light polarized in thestretched direction in the visible range (400-700 nm) shown by curve c.The film has an average transmission of 1.6% for curve c between 400 and700 nm. The % RMS color for curve a is 3.2%, while the % RMS color forcurve b is 18.2%.

EXAMPLE 12 (PET:Ecdel 601, Mirror)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered to the feedblock at arate of 75 pounds per hour and Ecdel 9967 (a thermoplastic elastomeravailable from Eastman Chemical) was delivered at a rate of 60 poundsper hour. The PET was on the skin layers. The feedblock method was usedto generate 151 layers which was passed through two multipliersproducing an extrudate of 601 layers. The multipliers had a nominalmultiplication ratio of 1.2 (next to feedblock) and 1.27. Two skinlayers at a total throughput of 24 pounds per hour were addedsymmetrically between the last multiplier and the die. The skin layerswere composed of PET and were extruded by the same extruder supplyingthe PET to the feedblock. The web was length oriented to a draw ratio ofabout 3.3 with the web temperature at about 205° F. The film wassubsequently preheated to about 205° F. in about 35 seconds and drawn inthe transverse direction to a draw ratio of about 3.3 at a rate of about9% per second. The film was then relaxed about 3% of its maximum widthin a heat-set oven set at 450° F. The finished film thickness was about0.0027 inches.

The film provided the optical performance as shown in FIG. 32.Transmission is plotted as curve a and reflectivity is plotted as curveb. The luminous reflectivity for curve b is 91.5%.

EXAMPLE 13 (PEN:CoPEN, 601, Antireflected Polarizer)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A Polyethylenenaphthalate (PEN) with an intrinsic viscosity of 0.54 dl/g (60 wt %Phenol plus 40 wt % dichlorobenzene) was delivered by on extruder at arate of 75 pounds per hour and the coPEN was delivered by anotherextruder at 65 pounds per hour. The coPEN was a copolymer of 70 mole %2.6 naphthalene dicarboxylate methyl ester, 30% dimethyl terephthalatewith ethylene glycol. The feedblock method was used to generate 151layers. The PEN skin layers were coextruded on the outside of theoptical stack with a total thickness of 8% of the coextruded layers. Thefeedblock was designed to make a linear gradient in layer thickness fora 149 layer optical stack with the thinnest layers on one side of thestack. The individual layer thicknesses were designed in pairs to makeequal thickness layers of the PEN and coPEN for each pair. Each pairthickness, d, was determined by the formula d=do+do*0.003*n, where do isthe minimum pair thickness, and n is the pair number between 1 and 75.The optical stack was multiplied by two sequential multipliers. Thenominal multiplication ratio of the multipliers were 1.2 and 1.27,respectively. The film was subsequently preheated to 320° F. in about 40seconds and drawn in the transverse direction to a draw ratio of about5.0 at a rate of 6% per second. The finished film thickness was about 2mils.

A silical sol gel coating was then applied to one side of the reflectingpolarizer film. The index of refraction of this coating wasapproximately 1.35. Two pieces of the AR coated reflecting polarizerfilm were cut out and the two were laminated to each other with the ARcoatings on the outside. Transmission spectra of polarized light in thecrossed and parallel directions were obtained. The sample was thenrinsed with a 2% solution of ammonium bifluoride (NH4 HF2) in deionizedwater to remove the AR coating. Spectra of the bare multilayer were thentaken for comparison to the coated sample.

FIG. 33 shows the spectra of the coated and uncoated polarizer. Curves aand b show the transmission and extinction, respectively, of the ARcoated reflecting polarizer, and curves c and d show the transmissionand extinction, respectively, of the uncoated reflecting polarizer. Notethat the extinction spectrum is essentially unchanged, but that thetransmission values for the AR coated polarizer are almost 10% higher.Peak gain was 9.9% at 565 nm, while the average gain from 425 to 700 runwas 9.1%. Peak transmission of the AR coated polarizer was 97.0% at 675nm. Average transmissions for curve a over 400-700 nm was 95.33%, andaverage transmission for curve d over 400-700 nm was 5.42%.

EXAMPLE 14 (PET:Ecdel, 601, Polarizer)

A coextruded film containing 601 layers was made on a sequentialflat-film-making line via a coextrusion process. A polyethyleneterephthalate (PET) with an Intrinsic Viscosity of 0.6 dl/g (60 wt. %phenol/40 wt. % dichlorobenzene) was delivered to a feedblock by oneextruder at a rate of 75 pounds per hour and Ecdel 9967 (a thermoplasticelastomer available from Eastman Chemical) was delivered to thefeedblock by another extruder at a rate of 60 pounds per hour. The PETwas on the skin layers. The feedblock method was used to generate 151layers which passed through two multipliers (2×) producing an extrudateof 601 layers. A side stream with a throughput of 50 pounds per hour wastaken from the PET extruder and used to add two skin layers between thelast multiplier and the die. The web was length oriented to a draw ratioof about 5.0 with the web temperature at about 210° F. The film was nottentered. The finished film thickness was about 2.7 mil.

FIG. 34 shows the transmission for this film. Curve a shows thetransmission of light polarized in the stretch direction, while curve bshows the transmission of light polarized orthogonal to the stretchdirection. The average transmission from 400-700 nm for curve a is39.16%.

EXAMPLE 15 (PEN:CoPEN, 449, Polarizers)

A coextruded film containing 449 layers was made by extruding the castweb in one operation and later orienting the film in a laboratoryfilm-stretching apparatus. A polyethylene naphthalate (PEN) with anIntrinsic Viscosity of 0.53 dl/g (60 wt. % phenol/40 wt. %dichlorobenzene) was delivered by one extruder at a rate of 26.7 poundsper hour to the feedblock and a different material was delivered bysecond extruder at a rate of 25 pounds per hour to the feedblock. ThePEN was the skin layers. The feedblock method was used to generate 57layers which passed through three multipliers producing an extrudate of449 layers. The cast web was 0.0075 mils thick and 12 inches wide. Theweb was later uniaxially oriented using a laboratory stretching devicethat uses a pantograph to grip a square section of film and stretch itin one direction at a uniform rate while it is constrained in the other.A 7.46 cm square of web was loaded into the stretcher at about 100° C.and heated to 140° C. for 60 seconds. Stretching then commenced at10%/sec (based on original dimensions) until the sample was stretched toabout 5.5×1. Immediately after stretching, the sample was cooled byblowing room temperature air at it.

The input to the second extruder was varied by blending pellets of thefollowing poly(ethylene esters) three materials: (i) a coPEN (70 mol %2.6-napthalene dicardoxylate and 30 mol % terephthalate) with anintrinsic viscosity of 0.52 (60 wt. % phenol/40 wt. % dichlorobenzene);(ii) the PEN, same material as input to first extruder: (iii) a PET,with an intrinsic viscosity of 0.95 (60 wt. % phenol/40 wt. %dichlorobenzene). TTF 9506 purchased from Shell.

For the film shown in FIG. 35A the input to the second extruder was80-wt % of the coPEN and 20 wt % of the PEN: for the film shown in FIG.35B the input to the second extruder was 80 wt % of the coPEN and 20 wt% of the PET: for the film shown in FIG. 35C the input to the secondextruder was coPEN.

FIGS. 35A, 35B, and 35C show the transmission of these multilayer filmswhere curve a shows transmission of light polarized in the non-stretchdirection at normal incidence, curve b shows transmission of p-polarizedlight polarized in the non-stretched direction at 60° incidence, andcurve c shows transmission of light polarized in the stretch directionat normal incidence. Note that the optical response of these films issensitive to the chemical composition of the layers from the secondextruder. The average transmission for curve c in FIG. 35A is 43.89%,the average transmission for curve c in FIG. 35B is 1.52%, and theaverage transmission for curve c in FIG. 35C is 12.48%. Thus, extinctionis increased from FIG. 35A to FIG. 35C.

For the examples using the 57 layer feedblock, all layers were designedfor only one optical thickness (¼ of 550 nm), but the extrusionequipment introduces deviations in the layer thicknesses throughout thestack resulting in a fairly broadband optical response. For examplesmade with the 151 layer feedblock, the feedblock is designed to create adistribution of layer thicknesses to cover a portion of the visiblespectrum. Asymmetric multipliers were then used to broaden thedistribution of layer thicknesses to cover most of the visible spectrumas described in U.S. Pat. Nos. 5,094,788 and 5,094,793.

Although the present invention has been described with reference topreferred embodiments, those of skill in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

We claim:
 1. A multilayered optical film, comprising: a plurality of alternating first and second optical layers; wherein the first optical layers comprise a first polymer selected from polyalkylene terephthalates, polyalkylene naphthalates, and copolymers of polyalkylene naphthalates and the first polymer has at least 0.05 difference in in-plane refractive indices; and wherein the second optical layers comprise a second polymer selected from copolyesters and copolycarbonates and the second polymer has isotropic in-plane refractive indices.
 2. The multilayered optical film of claim 1, wherein the first polymer has at least 0.20 difference in in-plane refractive indices.
 3. The multilayered optical film of claim 1, wherein the first polymer is a copolymer of a polyalkylene naphthalate formed using at least one comonomer selected from isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 4. The multilayered optical film of claim 1, wherein the first polymer is selected from polyethylene naphthalate, polybutylene naphthalate, copolymers of polyethylene naphthalate, and copolymers of polybutylene naphthalate.
 5. The multilayered optical film of claim 1, wherein the first polymer is selected from polyethylene naphthalate and copolymers of polyethylene naphthalate.
 6. The multilayered optical film of claim 5, wherein the first polymer is a copolymer of polyethylene naphthalate formed using at least one comonomer selected from phthalic, isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 7. The multilayered optical film of claim 5, wherein the first polymer has at least 0.20 difference in in-plane refractive indices.
 8. The multilayered optical film of claim 1, wherein the first polymer is polyethylene terephthalate.
 9. The multilayered optical film of claim 1, wherein the second polymer is selected from copolymers of polyalkylene naphthalates and copolymers of polyalkylene terephthalates.
 10. The multilayered optical film of claim 1, wherein the second polymer is a copolymer of polyethylene naphthalate.
 11. The multilayered optical film of claim 10, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from phthalic, isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 12. The multilayered optical film of claim 10, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from propane diol, butane diol, neopentyl glycol, polyethylene glycol, tetramethylene glycol, dimethylene glycol, cyclohexanedimethanol, 4-hydroxy diphenol, bisphenol A, 1,8-dihydroxy biphenyl, and 1,3-bis(2-hydroxyethoxy)benzene.
 13. The multilayered optical film of claim 1, wherein the second polymer is a copolymer of polyethylene terephthalate.
 14. The multilayered optical film of claim 1, wherein the first polymer is polyethylene naphthalate and the second polymer is a copolymer of polyethylene naphthalate.
 15. The multilayered optical film of claim 1, wherein the first polymer is a copolymer of polyethylene naphthalate and the second polymer is a copolymer of polyethylene naphthalate.
 16. The multilayered optical film of claim 1, wherein the first polymer is polyethylene naphthalate and the second polymer is a copolymer of polyethylene terephthalate.
 17. The multilayered optical film of claim 1, wherein the first polymer is polyethylene terephthalate and the second polymer is a copolymer of polyethylene naphthalate.
 18. The multilayered optical film of claim 1, wherein the second polymer is selected from a copolymer of polyethylene naphthalate with 70 mol % naphthalate and 30 mol % terephthalate and a copolymer of polyethylene naphthalate with 70 mol % naphthalate, 15 mol % terephthalate, and 15 mol % isophthalate.
 19. A polarizer, comprising: a plurality of alternating first and second optical layers; wherein the first optical layers comprise a first polymer selected from polyalkylene naphthalates and copolymers of polyalkylene naphthalates and the first polymer has at least 0.05 difference in in-plane refractive indices; and wherein the second optical layers comprise a second polymer selected from copolyesters and copolycarbonates and the second polymer has isotropic in-plane refractive indices.
 20. The polarizer of claim 19, wherein the first polymer has at least 0.20 difference in in-plane refractive indices.
 21. The polarizer of claim 19, wherein the first polymer is a copolymer of a polyalkylene naphthalate formed using at least one comonomer selected from isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 22. The polarizer of claim 19, wherein the second polymer is selected from copolymers of polyalkylene naphthalates and copolymers of polyalkylene terephthalates.
 23. The polarizer of claim 19, wherein the second polymer is a copolymer of polyethylene naphthalate.
 24. The polarizer of claim 23, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from phthalic, isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 25. The polarizer of claim 23, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from propane diol, butane diol, neopentyl glycol, polyethylene glycol, tetramethylene glycol, dimethylene glycol, cyclohexanedimethanol, 4-hydroxy diphenol, bisphenol A, 1,8-dihydroxy biphenyl, and 1,3-bis(2-hydroxyethoxy)benzene.
 26. The polarizer of claim 19, wherein the first polymer is a copolymer of polyethylene naphthalate and the second polymer is a copolymer of polyethylene naphthalate.
 27. A polarizer, comprising: a plurality of alternating first and second optical layers; wherein the first optical layers comprise a first polymer selected from copolymers of polyethylene naphthalate and the first polymer has at least 0.05 difference in in-plane refractive indices; and wherein the second optical layers comprise a second polymer selected from copolymers of polyethylene naphthalate.
 28. The polarizer of claim 27, wherein the first polymer has at least 0.20 difference in in-plane refractive indices.
 29. The polarizer of claim 27, wherein the first polymer is a copolymer of a polyalkylene naphthalate formed using at least one comonomer selected from isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 30. The polarizer of claim 27, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from phthalic, isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic, 2,7-naphthalene, 2,6-naphthalene, and cyclohexane dicarboxylic acids and esters thereof.
 31. The polarizer of claim 27, wherein the copolymer of polyethylene naphthalate is formed using at least one comonomer selected from propane diol, butane diol, neopentyl glycol, polyethylene glycol, tetramethylene glycol, dimethylene glycol, cyclohexanedimethanol, 4-hydroxy diphenol, bisphenol A, 1,8-dihydroxy biphenyl, and 1,3-bis(2-hydroxyethoxy)benzene. 